1. Field of the Invention
The present invention relates to an optical waveguide circuit, and more particularly to an optical waveguide circuit constructed by combining optical waveguides composed of a core and a cladding.
2. Description of the Related Art
A wavelength division multiplexing (WDM) transmission system, which assigns a plurality of optical signals to different wavelengths and transmits them through a single optical fiber, can greatly increase the capacity of the transmission. To configure an optical transmission network with the optical WDM systems, a wavelength multi/demultiplexer is necessary which multiplexes optical signals with different wavelengths into a single optical fiber, or demultiplexes the optical signals, which are multiplexed into a single optical fiber, to the original wavelengths.
In particular, an N×N cyclic wavelength multi/demultiplexer that has N input ports and N output ports, and has a demultiplexing characteristic with cyclic input/output relationship can have various applications in the optical WDM system.
FIG. 1 shows an example of the demultiplexing characteristics of the N×N cyclic wavelength multi/demultiplexer. Although it includes N2 paths between N input ports and N output ports, it can establish all the paths independently by only N wavelengths λ1-λN. The characteristics enable an N×N wavelength router function that establishes full mesh links among N nodes, or an N×N optical switch function by combining with N tunable wavelength light sources.
On the other hand, intensive research and development about planar lightwave circuits, which are constructed using silica-based glass waveguides formed on a silicon substrate, have been progressed. The planar lightwave circuit includes an arrayed-waveguide grating (AWG) that implements the wavelength multi/demultiplexing. The detail of the AWG is described in H. Takahashi et al., “Arrayed-Waveguide Grating for Wavelength Division Multi/Demultiplexer With Nanometer Resolution”, Electron. Lett., Vol. 26, No. 2, pp. 87-88, 1990.
FIG. 2 shows a circuit configuration of an AWG. It includes on a Si substrate 1, input waveguides 2, a first slab waveguide 3, arrayed-waveguides 4, a second slab waveguide 5 and output waveguides 6. The N×N cyclic wavelength multi/demultiplexer described before is implemented by a planar lightwave circuit composed of a combination of the AWG described above and optical couplers.
FIG. 3 is a diagram showing a configuration of a conventional N×N cyclic wavelength multi/demultiplexer using an AWG and optical couplers. In FIG. 3, the reference numeral 11 designates N input ports, 12 designates N output ports, 13 designates an AWG having a plurality of input and output waveguides, 14 designates a plurality of 2×1 optical couplers, and 15 designates a plurality of connecting waveguides that connect the AWG 13 to the optical couplers 14.
FIG. 3 shows an example in which the number of the input and output ports are four each, the multiplexed number of the wavelengths is four, and the number of each of the input and output waveguides of the AWG 13 is eight. Four quadplexed optical signals A1, A2, A3 and A4, B1, B2, B3 and B4, C1, C2, C3 and C4, and D1, D2, D3 and D4 are incident onto the AWG 13 through the four input ports 11 connected to the four input waveguides among the eight of the AWG 13.
Here, the alphabetical letters indicate the positions of the input ports, and the numbers indicate the wavelengths. Accordingly, the optical signals with the same alphabetical letter are inputted into the same input port, and the optical signals with the same number have the same wavelength. Therefore 16 different optical signals are transmitted here at four wavelengths. The input optical signals are wavelength demultiplexed to the output ports according to the characteristics of the AWG 13, and are supplied to the connecting waveguides 15 in such a manner that the optical signals supplied to each input waveguide are output from the consecutive output ports in accordance with their wavelengths.
Then, the 2×1 optical couplers 14 couple the waveguides of the connecting waveguides 15 corresponding to the output waveguides (2) and (6), (3) and (7), and (4) and (8) of the AWG 13, respectively. As a result, the N output ports 12 connected to the outputs of the 2×1 optical couplers 14 implement the demultiplexing characteristics of the cyclic input and output relationships as described in connection with FIG. 1.
To thus configuring the N×N cyclic wavelength multi/demultiplexer as a single planar lightwave circuit using the AWG and optical couplers, the connecting waveguides 15 that connect the AWG 13 with the optical couplers 14 must intersect with other connecting waveguides (N−1) times at the maximum. In the example of FIG. 3 where N=4, the waveguide corresponding to the output waveguide (4) or (5) of the AWG 13 intersects with the other three waveguides.
In addition, the waveguide corresponding to the output waveguide (3) or (6) of the AWG 13, and the waveguide corresponding to the output waveguide (2) or (7) intersect with other waveguides twice and once, respectively, and the waveguide corresponding to the output waveguide (8) does not intersect with other waveguides. Thus, the number of intersections with the other waveguides varies depending on the connecting waveguides.
As described above by way of example of the N×N cyclic wavelength multi/demultiplexer, the optical waveguide circuit like the foregoing planar lightwave circuit must have intersections of the waveguides to implement a more complicated waveguide layout, or to interconnect a plurality of component circuits in combination.
The waveguide intersections, however, cause a loss. Accordingly, a complicated waveguide layout or interconnections of a lot of component circuits in combination, which include many intersections, can increase the loss of the circuit because of cumulative losses, thereby degrading the circuit characteristics, and making the practical use of the circuit difficult.
In particular, configuring the N×N cyclic wavelength multi/demultiplexer as a single planar lightwave circuit brings about (N−1) waveguide intersections at the maximum for the circuit scale N. In addition, the respective connecting waveguides can have different numbers of the intersections. Accordingly, the loss increase of the circuit is inevitable when realizing a large-scale circuit. Furthermore, difference of the input and output ports and differences of wavelength of the optical signal passing through the circuit can cause great loss difference.
Therefore the conventional cyclic wavelength multi/demultiplexer has a problem in that it is difficult to realize a circuit which has a large circuit scale in terms of the number of the multiplexed wavelengths or that of the input and output ports, with achieving a good loss characteristics.